Different processes are known for protection of metallic surfaces against corrosion, in particular of surfaces of steel components.
Today, zinc coating of steel surfaces is one of the most effective methods for protection of steel components against general corrosion. Zinc coatings are widely used in the mass production of tubes, chassis components and the like. In a corrosive medium, the zinc coating or zinc layers serve as sacrificial anode (at good electrical contact with the steel surfaces) due to the less precious character of zinc compared to iron. Therefore, the zinc layer corrodes first before the iron is exposed to corrosion and forms red-brown corrosion products, which are also referred to as red rust. Pure zinc shows a relatively high corrosion speed in many corrosive media while forming voluminous white rust products, which are also referred to as white rust.
The zinc corrosion, however, may be considerably delayed by appropriate alloys and/or by suitable post treatment of the zinc layer. The latter case mainly is a so called passivation process. In such process a thin inorganic protective layer is produced on a zinc layer surface which is treated with passivation solution. This layer is also referred to as conversion layer, if cations of at least one component of the metallic surface are extracted and also used for the layer formation. This layer increases the corrosion resistance of the zinc coatings and thus the corrosion resistance of the zinc coated steel components.
The passivation processes may be used for galvanic zinc coated parts as well as for hot-dip galvanized parts. In particular for galvanic zinc coated parts different chromating processes have been developed. By treatment with a corresponding chromium-containing passivation solution, depending on the composition of this solution, clear, blue, yellow, olive green or black conversion layers form, which differ from each other in their protection value.
The passivation layer primarily serves for temporary corrosion protection of the zinc coated metal surface, for example tube surface. Due to handling, transport, storage or further processing of the tubes such a layer, however, may easily be harmed, which leads to a deterioration of the white rust resistance.
During further processing the zinc coated and passivated or chromated components, in particular tubes, are often subjected to strong forming, which exceeds the deformability of the conversion layer. During such forming unavoidably numerous cracks occur in the conversion layer due to its brittleness, which leads to a considerable deterioration of the white rust resistance or red rust resistance of the tube surface.